1. Field of the Invention
The present invention relates to a lighting apparatus and a control method thereof.
2. Description of the Related Art
In recent years, image display apparatuses using liquid crystals are becoming mainstream. Because liquid crystal panels are not self-luminous devices, a backlight using light sources such as LEDs (Light-Emitting Diodes) is used. In addition, in liquid crystal displays, as methods for adjusting the brightness of an image, there are known a method of adjusting brightness by means of a liquid crystal, and a method of changing the brightness of a backlight. In order to enhance a contrast ratio within a screen, it is better to use a method of adjusting the highest brightness by means of a backlight. PWM (Pulse Width Modulation) is well used as a method of adjusting the brightness of the backlight. This method is to adjust the brightness of the backlight by turning it on and off at a fixed period or cycle and changing a ratio between a turn-on period and a turn-off period (a duty ratio) of the backlight. When the period or cycle of turn-on and turn-off is long, blinking of light will be recognized by human eyes, and so a viewer may feel a flicker. For that reason, it is general to turn the backlight on and off at a high frequency equal to or higher than 200 Hz.
The backlight using LEDs is covered with many LEDs. The number of LEDs used in the backlight varies according to the size, required brightness, etc., of a display screen. Here, let us assume that the screen can be divided into 12 areas, as shown in FIG. 2A and FIG. 2B, and that light emission control on the LEDs can be carried out for each of the areas. In FIG. 15, there is shown a change in an amount of electric current in the power supply of the backlight at the time of carrying out PWM control in which LEDs in all the areas are caused to turn on and off at the same timing. In FIG. 15, an axis of abscissa represents time, and an axis of ordinate represents the amount of electric current. A sum of amounts of electric currents in all the individual areas provides a total current. As shown in FIG. 15, when PWM control is carried out at the same timing in all the areas, a total amount of current will vary to a large extent depending on time, in one cycle of PWM. Thus, when the amount of electric current varies to a large extent, a reduction in power supply efficiency will be caused, leading to an increase in the electric power consumption. In addition, it is necessary to design the power supply of the backlight so as to be able to withstand large variation, which leads to high cost.
When a method of turning on the LEDs while shifting the timing (phase) of PWM for each area (see, for example, Japanese patent application laid-open No. 2010-153359) is used, it is possible to suppress a variation in the total amount of current.
However, in a liquid crystal display, when displaying black, the light of a backlight is shaded or shielded by closing a liquid crystal shutter. However, the shading or shielding of the light can not be done to a sufficient extent by means of the liquid crystal shutter, and even if the liquid crystal shutter is closed, a slight amount of light leaks therethrough, so that there may occur so-called “black floating” (misadjusted black level or graying of black) in which sufficient black expression is not able to be carried out. Accordingly, when the brightness of the backlight is controlled so as to be uniform over the full screen, a contrast ratio in the screen will be limited by the black floating. As a technology for eliminating this, there exists one called local dimming. This is a technology of carrying out control such that the brightness of the backlight is made different for each area in the screen according to the brightness and darkness of an image (see, for example, Japanese patent application laid-open No. 2001-142409). By carrying out local dimming, it becomes possible to suppress the black floating and to enhance the contrast ratio in the screen.